The invention will be discussed in the context of trains for convenience, although it will be appreciated it is applicable to other systems, such as monorails the like, wherein a vehicle moves along a guideway.
As trains become more automated, there is a need to determine their position with a high degree of accuracy. This is particularly the case when the train stops at a platform, for example.
One system currently in use is known as the Transcore™ system as used in Seltrac. This employs transponders spaced along the track and a detector, known as an interrogator, on the train. Each train is controlled by a VOBC (or Very Intelligent On-board Controller). As the train moves past a transponder it picks up signals that provide two data outputs, namely a serial channel indicating the ID of the transponder and a digital output indicating when the interrogator is in communication with a transponder. The area above the transponder in the time domain where communication is possible is known as the transponder's footprint.
The Transcore system as used in Seltrac is based on matching the timing of the transponder's ID to the transponder's known position. However, this leads to a nominal positioning accuracy of +/−400 mm, which is insufficient for some purposes, such as accurately stopping of a train in a station.
A possible solution, which does not form part of the prior art, would be to observe the footprint and bisect it in order to get better positioning accuracy. This approach would be better than the currently implemented solution; however it assumes that the footprint, known in the Transcore system as TI Lock signal, is symmetrical about the physical centre of the transponder. Unfortunately, this is not exactly the case. The TI Lock centre may deviate by −322 to +390 mm in the nominal case from the transponder's physical centre. This is called the “Centre Deviation”. These values were observed during testing, so they could be higher in other situations. Using a three-sigma approach assuming normal distribution, 99.7% of centre deviations would be between −331 and 329 mm. There is also the possibility that electromagnetic interference can distort the TI lock signal.